1. Field of the Invention
The present invention relates to a transponder, in particular an antenna system for a transponder and activation or driver circuits incorporated in the transponder for issuing information carrying electromagnetic waves on the antenna system.
2. Description of the Prior Art
Electronic systems for identification of domestic animals utilize electronic units attached to the animals, usually in the shape of transponders that lack an energy source of their own. Such transponders obtain induced electric energy from receiving suitable electromagnetic energy that is emitted by antennas or coils located at those positions where an identification is desired, such as at the entrance of a feeding place or a milking station. After receiving induced energy the transponder issues a radio frequency signal that is captured by an antenna which can be the same antenna by means of which the energy was induced or at least is arranged in parallel therewith. The identification signal can then be used for e.g. only counting the number of animals, controlling amounts of delivered food, guiding a milking animal to the correct stall, etc.
Transponders are generally used for identification of movable objects, for example and primarily of bovine animals. A transponder comprises conventionally a receiving and transmitting antenna, a storage means for storing energy transferred wirelessly to the transponder and control circuits. A transponder is associated with at least one usually fixed or stationary reading station that comprises an antenna loop having one or more turns located at some position, in the direct neighbourhood of which the movable object is sometimes located or passes, such as for example directly through the antenna loop of the reading station. The antenna of the transponder is in a similar way made as a coil having a multitude of turns, the coil being substantially flat. The antenna of the transponder and the antenna in the reading station cooperate in principally the same way as the two windings of a transformer having an air core. By suitable arrangements it can often be achieved that these two coils at least sometimes will be located approximately in parallel with each other, when the movable object is staying close to or passes the stationary reading station, but this can be difficult to achieve in a secure manner, e.g. for the case where the reading station comprises loops located at each side of a passage through which the movable objects pass. Thus, it would be advantageous that the antenna system comprising the two antennas could be arranged for arbitrary mutual directions of the magnetic field of the stationary antenna in the reading station and the antenna of the transponder.
U.S. Pat. Nos. 4,247,758 and 4,798,175 and the published European patent applications EP-A1 0 494 764 and EP-A2 0 496 610 disclose typical examples of prior transponder systems and antennas designed therefor.
In U.S. Pat. No. 4,274,089 it is disclosed how an interrogation system for theft protection responders can be designed to have two transmission antennas, each one of which being a flat coil and which are located perpendicularly to each other. These antenna coils are made having only one terminal and thus they have one end open. They are supplied with electrical energy alternatingly so that always only one antenna is active. A separate receiving antenna is used. By the arrangement comprising antenna coils arranged perpendicularly to each other the station is less sensitive to the orientation of the antenna of a responder attached to some object that passes the interrogation station.
This previously known design having two antenna elements arranged perpendicularly to each other can of course also be modified for use in reading stations for transponders, as appears from the published European patent application EP-A1 0 496 609. Here two vertical antenna coils that are perpendicular to each other are provided surrounding the path along which a transponder passes. U.S. Pat. No. 5,258,766 discloses a similar way of accomplishing that the antenna system in a transponder/reading station installation will have the quality mentioned above, i.e. that a good transfer of energy and of information is obtained for arbitrary angles of the two antennas. The reading station is here provided with three antenna loops located perpendicularly to each other. In addition, each antenna loop consists of two parallel portions located at a distance from each other so that a cubical or complex, ball shaped arrangement is obtained, where in the cubical arrangement antenna conductors are located at all edge lines of the cube.
Antennas of reading stations are usually made in the shape of a portal structure, see for example the patent U.S. Pat. No. 4,798,175 cited above. The electrical conductor in the antenna here forms loops located at each side of a passage through which the animal are intended to pass. Such a portal structure could be completed to comprise an antenna arrangement that is previously known according to the discussion above having antenna loops located above and underneath the passage and further having loop elements through which the animal must pass, in order to produce a more secure transfer of energy and more secure receiving of information. However, the antenna arrangement must be made very sturdy or robust having the electrical conductors of the antenna attached to some electrically isolating material, that in the prior design comprises curtains suspended at the sides of the passage and consisting of thick plastic plates. The plastics material must here be selected to have such a thickness and such a quality that it is opaque, and thus the portal will have the appearance of a dark passage or dark opening. Bovine animals can be afraid of such a dark passage and special arrangements must be made for making them pass through the portal. If the prior portal is completed to comprise further parts it would be even clumsier and heavier and it must be designed to include rigid steel parts provided with a suitable electrical isolation. For a reading station for bovine animals where it is desired that the animals to which the transponders are attached at their passage preferably are to pass through or close to an antenna loop of the reading station, the antenna loop should thus be designed in the simplest possible way, having the most possible open configuration. Apparently, such an open configuration cannot be obtained using the prior antenna systems.
A transponder comprises conventionally an oscillatory circuit, in which the antenna coil and a capacitor are included and which is coupled to a capacitor for storing energy, see the published European patent application EP-A1 0 301 127 and the German patent documents DE-C1 40 04 801 and DE-C1 40 04 196. The oscillatory circuit is tuned to the frequency of the electromagnetic energy that is issued from a reading station together with which the transponder is intended to be used, and the oscillatory circuit is through some suitable rectifying device further coupled to a capacitor for storing energy. When sufficient energy has been received and stored in the storage capacitor, it provides supply voltage to control circuits so that information can be issued from the transponder antenna, generally having the same frequency as the received electromagnetic energy. Then the oscillatory circuit can be made to oscillate at its natural or resonant frequency by connecting it to the electrodes of the storage capacitor during short periods at suitable times, see the European patent application EP-A1 0 301 127 cited above. The times can be selected so that they occur at every second cycle of the natural oscillation of the oscillatory circuit, see the divider circuit 158 and the monostable flipflop 192 in FIG. 2. Information from the transponder is then emitted by modulating the transmitted wave in an off-and-on fashion. In order to obtain a safe reception it is required that the reading station issues the high frequency energy in intervals or periodically, so that after a transmission interval a transponder that is located within a certain distance from the antenna reading distance normally has had its storage capacitor charged to a sufficient voltage in order to drive its control circuits and in order to be able to transmit information.
In another design the information issued from the transponder can be carried by an electromagnetic oscillating wave having a frequency different from that of the received high frequency energy, see the German "Offenlegungsschrift" DE-A1 42 13 065. Thereby possibly high frequency can be transmitted more continuously and perhaps longer transmission periods of the transponder can be obtained.
However, a disadvantage in these known designs is that in emitting information from the transponder the modulation is made by means of an "on/off" process which is known to be sensitive to interference and can require several retransmissions of information in order that it will be safely received.